Soft Materials Research Articles

Ion‐Specific Interactions Engender Dynamic and Tailorable Properties in Biomimetic Cationic Polyelectrolytes

Biomaterials such as spider silk and mussel byssi are fabricated by the dynamic manipulation of intra‐ and intermolecular biopolymer interactions. Organisms modulate solution parameters, such as pH and ion co‐solute concentration, to effect these processes. These biofabrication schemes provide a conceptual framework to develop new dynamic and responsive abiotic soft material systems. Towards these ends, the chemical diversity of readily available ionic compounds offers a broad palette to manipulate the physicochemical properties of polyelectrolytes via ion‐specific interactions. In this study, we show for the first time that the ion‐specific interactions of biomimetic polyelectrolytes engenders a variety of phase separation behaviors, creating dynamic, thermal and ion responsive soft matter. that exhibits a spectrum of physical properties, spanning viscous fluids, to viscoelastic and viscoplastic solids. These ion dependent characteristics are further rendered general by the merger of lysine and phenylalanine into a single, amphiphilic vinyl monomer. The unprecedented breadth, precision, and dynamicity in the reported ion dependent phase behaviors thus introduce a broad array of opportunities for the future development of responsive soft matter, properties that are poised to drive developments in critical areas such as chemical sensing, soft robotics, and additive manufacturing.

Real-Space Visualization of Charged Polymer Network of Hydrogel by Double Network Strategy and Mineral Staining.

Hydrogels consist of three-dimensional (3D) and complicated polymer networks that determine their physical properties. Among the methods for structural analyses of hydrogels, the real-space imaging of a polymer network of hydrogels on a nanometer scale is one of the optimal methods; however, it is highly challenging. In this study, we propose a direct observation method for cationic polymer networks using transmission electron microscopy (TEM). By combining the double network strategy and the mineral staining technique, we overcame the challenges of polymer aggregation and the low electron density of the polymer. An objective cationic network was incorporated into a neutral skeleton network to suppress shrinkage during subsequent staining. Titania mineralization along the cationic polymer strands provided sufficient electron density for the objective polymer network for TEM observation. This observation method enables the visualization of local structures in real space and plays a complementary role to scattering methods for soft matter structure analysis.

Biomimetic Octopus Suction Cup with Attachment Force Self-Sensing Capability for Cardiac Adhesion.

This study develops a biomimetic soft octopus suction device with integrated self-sensing capabilities designed to enhance the precision and safety of cardiac surgeries. The device draws inspiration from the octopus's exceptional ability to adhere to various surfaces and its sophisticated proprioceptive system, allowing for real-time adjustment of adhesive force. The research integrates thin-film pressure sensors into the soft suction cup design, emulating the tactile capabilities of an octopus's sucker to convey information about the contact environment in real time. Signals from sensors within soft materials exhibiting complex strain characteristics are processed and interpreted using the grey wolf optimizer-back propagation (GWO-BP) algorithm. The tissue stabilizer is endowed with the self-sensing capabilities of biomimetic octopus suckers, and real-time feedback on the adhesion state is provided. The embedding location of the thin-film pressure sensors is determined through foundational experiments with flexible substrates, standard spherical tests, and biological tissue trials. The newly fabricated suction cups undergo compression pull-off tests to collect data. The GWO-BP algorithm model accurately identifies and predicts the suction cup's adhesion force in real time, with an error rate below 0.97% and a mean prediction time of 0.0027 s. Integrating this technology offers a novel approach to intelligent monitoring and attachment assurance during cardiac surgeries. Hence, the probability of potential cardiac tissue damage is reduced, with future applications for integrating intelligent biomimetic adhesive soft robotics.

An open-source membrane stretcher for simultaneous mechanical and structural characterizations of soft materials and biological tissues

The ability to simultaneously measure material mechanics and structure is central for understanding their nonlinear relationship that underlies the mechanical properties of materials, such as hysteresis, strain-stiffening and -softening, and plasticity. This experimental capability is also critical in biomechanics and mechanobiology research, as it enables direct characterizations of the intricate interplay between cellular responses and tissue mechanics. Stretching devices developed over the past few decades, however, do not often allow simultaneous measurements of the structural and mechanical responses of the sample. In this work, we introduce an open-source stretching system that can apply uniaxial strain at a submicron resolution, report the tensile force response of the sample, and be mounted on an inverted microscope for real-time imaging. Our system consists of a pair of stepper-based linear motors that stretch the sample symmetrically, a force transducer that records the sample tensile force, and an optically clear sample holder that allows for high-magnification microscopy. Using polymer samples and cellular specimens, we characterized the motion control accuracy, force measurement robustness, and microscopy compatibility of our stretching system. We envision that this uniaxial stretching system will be a valuable tool for characterizing soft and living materials.

Ion-Mediated Structural Discontinuities in Phospholipid Vesicles.

Despite intense research, methods for controlling soft matter's spontaneous self-assembly into well-defined layers remain a significant challenge. We observed ion-induced structural discontinuities of phospholipid vesicles that can be exploited for controlled self-assembly of soft materials, using DOPC and NaCl as a model system. The observations were made for the 0.25 wt % lipid concentration. We used dynamic light scattering, zeta-potential measurement, cryo-electron microscopy, small-angle X-ray, and small-angle neutron scattering to understand the reason for the discontinuities. For salt concentrations below 8 mM, we observed a decrease in the liposome diameter with increased NaCl concentration. Above 8 mM, we measured a discontinuity; the radius increases within a very narrow salt concentration range within less than 0.1 mM and then decreases for values greater than 8 mM. At 75 mM, the radius becomes constant until it grows again at around 500 mM. Microscopy and scattering experiments show a transition from unilamellar to bilamellar at 8 mM and to trilamellar at 75 mM. At 500 mM, we found a heterogeneous liposome system with many different bilayer numbers. All the experimental observations indicate that declining solvent quality and increasing osmotic pressure direct lipids to expel preferentially to the inner compartment. Upon reaching a critical concentration, excess lipids can form a new bilayer. This spontaneous self-assembly process causes simultaneous shrinkage of the aqueous core and expansion of the vesicle. This approach opens an intriguing path for controlling the self-assembly of bioinspired colloids.

Pushing with Soft Robotic Arms via Deep Reinforcement Learning

Soft robots can adaptively interact with unstructured environments. However, nonlinear soft material properties challenge modeling and control. Learning‐based controllers that leverage efficient mechanical models are promising for solving complex interaction tasks. This article develops a closed‐loop pose/force controller for a dexterous soft manipulator enabling dynamic pushing tasks using deep reinforcement learning. Force tests investigate the mechanical properties of a soft robot module, resulting in orthogonal forces of N. Then, the policy is trained in simulation leveraging a dynamic Cosserat rod model of the soft robot. Domain randomization mitigate the sim‐to‐real gap while careful reward engineering induced pose and force control even without explicit force inputs. Despite the approximate simulation, the sim‐to‐real transfer achieved an average reaching distance of mm (), an average orientation error of rad () and applied pushing forces up to N. Such performance is reasonable for the intended assistive tasks of the manipulator. The experiments uncovered that the soft robot interacting with the environment exhibited torsional and counter‐balancing movements. Although not explicitly enforced, they emerged from the mechanical intelligence of the manipulator. The results demonstrate the potential of soft robotic manipulation via reinforcement learning.

Liquid Metal Chameleon Tongues: Modulating Surface Tension and Phase Transition to Enable Bioinspired Soft Actuators

Leveraging the unique attributes of functional soft materials to generate force and deformation, significant advancements in soft actuators are driving the evolution of smart robotics. Liquid metals (LMs), known for their high deformability and tunable morphology, demonstrate remarkable actuating capabilities through controllable surface tension. Inspired by the predation method of chameleons, this work introduces a bioinspired LM actuator (BLMA) by modulating the morphology of LM. This BLMA enables high‐strain (up to 170%) actuation by precisely directing LM droplets toward an electrode. Various parameters affecting the BLMA's actuating performance are explored. Notably, the application of a reductive voltage induces rapid solidification of supercooled LM, facilitating phase transition at room temperature. The solidified LM enhances its holding force of BLMA by over 1000 times. To underscore the superior capabilities of the BLMA, diverse applications, such as a complex two‐dimensional plane actuator, a stepper motor with adjustable step intervals, a phase transition‐controlled relay, and a laser code lock actuation gate set, are presented. It is anticipated that the exceptional characteristics of the BLMA will propel advancements in the realms of soft robotics and mechatronics.

Subsurface Profiling of Ion Migration and Swelling in Conducting Polymer Actuators with Modulated Electrochemical Atomic Force Microscopy.

Understanding the dynamics of ion migration and volume change is crucial to studying the functionality and long-term stability of soft polymeric materials operating at liquid interfaces, but the subsurface characterization of swelling processes in these systems remains elusive. In this work, we address the issue using modulated electrochemical atomic force microscopy as a depth-sensitive technique to study electroswelling effects in the high-performance actuator material polypyrrole doped with dodecylbenzenesulfonate (Ppy:DBS). We perform multidimensional measurements combining local electroswelling and electrochemical impedance spectroscopies on microstructured Ppy:DBS actuators. We interpret charge accumulation in the polymeric matrix with a quantitative model, giving access to both the spatiotemporal dynamics of ion migration and the distribution of electroswelling in the electroactive polymer layer. The findings demonstrate a nonuniform distribution of the effective ionic volume in the Ppy:DBS layer depending on the film morphology and redox state. Our findings indicate that the highly efficient actuation performance of Ppy:DBS is caused by rearrangements of the polymer microstructure induced by charge accumulation in the soft polymeric matrix, increasing the effective ionic volume in the bulk of the electroactive film for up to two times the value measured in free water.

Soft pads for robotic limbs: achieving human finger compliance via finite element optimization

Human fingers possess compliant contact interfaces with predetermined compliance properties. Replicating these compliance properties in robotic limbs, particularly anthropomorphic hands, can significantly impact grasp stability and contact area. To address the limitations of homogeneous layers of soft material commonly used in anthropomorphic hands, this paper proposes optimizing differentiated structures to design soft pads, which consist of a continuous external layer (skin) coupled with an internal layer featuring voids. To achieve the desired compliance properties replicating those of human fingers, the paper introduces five distinct patterns for differentiated structures, along with their corresponding finite element analysis (FEA) models. Subsequently, by formulating an appropriate optimization function, the shapes of these patterns have been optimized to match the compliance of the human finger, and it has been demonstrated that one of the proposed and optimized pads closely approximates this target through the proposed methodology. The results affirm that this approach offers an effective solution for designing soft pads whose behavior needs to be aligned with the compliance of a specific target.

MERCI: Mixed curvature-based elements for computing equilibria of thin elastic ribbons

Thin elastic ribbons represent a class of intermediary objects lying in-between thin elastic plates and thin elastic rods. Although the two latter families of thin structures have received much interest from the Computer Graphics community over the last decades, ribbons have seldom been considered and modelled numerically so far, in spite of a growing number of applications in Computer Design. In this paper, starting from the reduced developable ribbon models [Sadowsky 1929; Wunderlich 1962] recently popularised in Soft Matter Physics, we propose a both accurate and efficient algorithm for computing the statics of thin elastic ribbons. Inspired by the super-clothoid model for thin elastic rods, our method relies on compact ribbon elements whose normal curvature varies linearly with respect to arc length s , while their geodesic torsion is quadratic in s . In contrast however, for the sake of efficiency, our algorithm avoids building a fully reduced kinematic chain and instead treats each element independently, gluing them only at the final solving stage through well-chosen bilateral constraints. Thanks to this mixed variational strategy, which yields a banded Hessian, our algorithm recovers the linear complexity of low-order models while preserving the quadratic convergence of curvature-based models. As a result, our approach is scalable to a large number of elements, and suitable for various boundary conditions and unilateral contact constraints, making it possible to handle challenging scenarios such as confined buckling experiments or Möbius bands with contact. Remarkably, our mixed algorithm proves an order of magnitude faster compared to Discrete Elastic Ribbon models of the literature while achieving, in a few seconds only, high accuracy levels that remain out of reach for such low-order models. Additionally, our numerical model can incorporate various ribbon energies, including the RibExt model for quasi-developable ribbons recently introduced in Physics [Audoly and Neukirch 2021], which allows to transition smoothly between a rectangular Kirchhoff rod and a (developable) Sadowsky ribbon. Our numerical scheme is carefully validated against demanding experiments of the Physics literature, which demonstrates its accuracy, efficiency, robustness, and versatility. Our Merci code is publicly available at https://gitlab.inria.fr/elan-public-code/merci for the sake of reproducibility and future benchmarking.

Overview of Dynamic Bond Based Hydrogels for Reversible Adhesion Processes.

Polymeric hydrogels are soft materials with a three-dimensional (3D) hydrophilic network capable of retaining and absorbing large amounts of water or biological fluids. Due to their customizable properties, these materials are extensively studied for developing matrices for 3D cell culture scaffolds, drug delivery systems, and tissue engineering. However, conventional hydrogels still exhibit many drawbacks; thus, significant efforts have been directed towards developing dynamic hydrogels that draw inspiration from organisms' natural self-repair abilities after injury. The self-healing properties of these hydrogels are closely associated with their ability to form, break, and heal dynamic bonds in response to various stimuli. The primary objective of this review is to provide a comprehensive overview of dynamic hydrogels by examining the types of chemical bonds associated with them and the biopolymers utilized, and to elucidate the chemical nature of dynamic bonds that enable the modulation of hydrogels' properties. While dynamic bonds ensure the self-healing behavior of hydrogels, they do not inherently confer adhesive properties. Therefore, we also highlight emerging approaches that enable dynamic hydrogels to acquire adhesive properties.

Counteraction of Russian Finance Market Against External Sanctions Pressure

The article shows the impact of external economic sanctions on the Russian financial market. Special attention is paid to the currency, commodity and derivative markets of the Moscow Stock Exchange. The main focus of the work is the analysis of changes in market dynamics and pricing under the influence of political and economic constraints. Methodologically, the study is based on a comprehensive analysis of trade session data, including statistical processing and modeling of market reactions. The use of actual analytical methods made it possible to identify nonlinear dependencies and hidden patterns of behavior of market participants. The results of the study demonstrate that sanctions have had a significant impact on volatility, liquidity and market structure. The authors show the influence of political factors on financial instruments in a crisis, as well as develop new methodological approaches to analyzing financial market data. The results of the work can be used to coordinate trading strategies, develop market regulation policies and assess risks in conditions of political instability. The study also provides a basis for further analysis of the consequences of economic sanctions and the development of strategies to adapt to changing market conditions.

Adaptive phase-field total Lagrangian material point method for evaluating dynamic fracture of soft material

Adaptive phase-field total Lagrangian material point method for evaluating dynamic fracture of soft material

Variational approaches to the elasticity of deformable strings with and without mass redistribution

AbstractThe catenary is a curve that has played a significant role in the history of mathematics, finding applications in various disciplines such as mechanics, technology, architecture, the arts, and biology. In this paper, we introduce some generalizations by applying the variational method to deformable strings. We explore two specific cases: (i) in the first case, we investigate the nonlinear behavior of an elastic string with variable length, dependent on the applied boundary conditions; specifically, this analysis serves to introduce the variational method and demonstrate the process of finding analytical solutions; (ii) in the second case, we examine a deformable string with a constant length; however, we introduce mass redistribution within the string through nonlinear elastic interactions. In the first scenario, the deformation state of the string always describes elongation, as compression states prove to be unstable for fully flexible strings. In contrast, in the second scenario, the finite length constraint induces compressive states in specific configurations and regions of the string. However, it is worth noting that the solution to this problem exists only for values of the elastic constant that are not too low, a phenomenon that is studied in detail. We conduct here both analytical and graphical analyses of various geometries, comparing the elastic behavior of the two aforementioned types of strings. Understanding the elastic behavior of deformable strings, especially the second type involving mass redistribution, is crucial for enhancing comprehension in the study of biological filaments or fibers and soft matter. For instance, these investigations can contribute to understanding the mechanisms employed by cells to sense gravity or other mechanical conditions.

Dimension reduction and homogenization of composite plate with matrix pre-strain

This paper focuses on the simultaneous homogenization and dimension reduction of periodic composite plates within the framework of non-linear elasticity. The composite plate in its reference (undeformed) configuration consists of a periodic perforated plate made of stiff material with holes filled by a soft matrix material. The structure is clamped on a cylindrical part. Two cases of asymptotic analysis are considered: one without pre-strain and the other with matrix pre-strain. In both cases, the total elastic energy is in the von-Kármán (vK) regime ( ε 5 ). A new splitting of the displacements is introduced to analyze the asymptotic behavior. The displacements are decomposed using the Kirchhoff–Love (KL) plate displacement decomposition. The use of a re-scaling unfolding operator allows for deriving the asymptotic behavior of the Green St. Venant’s strain tensor in terms of displacements. The limit homogenized energy is shown to be of vK type with linear elastic cell problems, established using the Γ-convergence. Additionally, it is shown that for isotropic homogenized material, our limit vK plate is orthotropic. The derived results have practical applications in the design and analysis of composite structures.

Analysis of Ballistic Capability in Making Bulletproof Multilayers Using Woven Ramie Fiber, Hardfacing Metal with Epoxy Matrix for Bulletproof Vests

Bulletproof material in level III bulletproof vests must be created using hard and soft materials to prevent bullet penetration. Kevlar and SiC + PE are imported materials used as armor in bulletproof vests. The use of Kevlar and SiC is very dependent on foreign sources with high prices. An innovation was carried out by making bulletproof material using cheaper multilayer methods involving metal from hardfacing welding as a hard layer and ramie fiber as a soft layer with an epoxy matrix. To see the level of penetration and surface morphology structure in the specimens, 9 variations were made: 3 layers of hardfacing welding and 3 epoxy volume fractions (40%, 45%, 50%). Ballistic test using NIJ 0101.06 level III standards with AK-47 of 7.62 mm x 39 mm bullets and the morphology of bulletproof multilayer material after impact was observed using SEM. The results showed all specimens failed to withstand bullet penetration at a distance of 15 meters. Meanwhile, at 50 meters, the S2-C specimen with a 60% epoxy volume fraction was able to withstand bullet penetration with a BFS value of 25.85 mm. This value is < 44 mm; thus, it complies with the NIJ 0101.06 body armor standard. According to SEM, most of the failures occurred because the resulting adhesion force was weak. It was necessary to add reinforcing material so that the adhesion force between the metal, ramie fiber, and adhesive increases; the materials must also have very light density to prevent the effectiveness reduction of the multilayer.

Promoting stimuli-responsive motion in soft matter by host-guest interactions

Promoting stimuli-responsive motion in soft matter by host-guest interactions

Connected three-dimensional polyhedral frames for programmable liquid processing

Human civilization relies heavily on the ability to precisely process liquids. Switching between liquid capture and release plays a fundamental role in the handling of various liquids, with applications that demand reversible, spatially and temporally precise, volumetrically accurate and programmable control over the liquid, independent of the details of the employed solid tools and processed liquids. However, current fluidic techniques do not fully meet these requirements. Here we present connected polyhedral frames to effectively address this challenge by tailoring liquid continuity between frames to dictate the liquid capture or release of individual frames, with an overall network that is readily switchable locally, dynamically and reversibly. Each frame captures or releases liquids, independent of its base materials, structures and processed liquids. The connected polyhedral frames are a versatile tool that enables many important functions including three-dimensional (3D) programmable patterning of liquids, 3D spatiotemporal control of concentrations of multiple materials, packaging of 3D liquid arrays and large-scale manipulation of multiple liquids, thus considerably advancing many fields, including interface science and soft materials.

Spatial organization of the ions at the free surface of imidazolium-based ionic liquids

HypothesisExperimental information on the molecular scale structure of ionic liquid interfaces is controversial, giving rise to two competing scenarios, namely the double layer-like and “chessboard”-like structures. This issue can be resolved by computer simulation methods, at least for the underlying molecular model. Systematically changing the anion type can elucidate the relative roles of electrostatic interactions, hydrophobic (or, strictly speaking, apolar) effects and steric restrictions on the interfacial properties. SimulationsMolecular dynamics simulation is combined with intrinsic analysis methods both at the molecular and atomic levels, supplemented by Voronoi analysis of self-association. FindingsWe see no evidence for the existence of a double-layer-type arrangement of the ions, or for their self-association at the surface of the liquid. Instead, our results show that cation chains associate into apolar domains that protrude into the vapour phase, while charged groups form domains that are embedded in this apolar environment at the surface. However, the apolar chains largely obscure the cation groups, to which they are bound, while the smaller and more mobile anions can more easily access the free surface, leading to a somewhat counterintuitive net excess of negative charge at the interface. Importantly, this excess charge could only be identified by applying intrinsic analysis.

Minimal actuation and control of a soft hydrogel swimmer from flutter instability

Micro-organisms propel themselves in viscous environments by the periodic, nonreciprocal beating of slender appendages known as flagella. Active materials have been widely exploited to mimic this form of locomotion. However, the realization of such coordinated beating in biomimetic flagella requires complex actuation modulated in space and time. We prove through experiments on polyelectrolyte hydrogel samples that directed undulatory locomotion of a soft robotic swimmer can be achieved by untethered actuation from a uniform and static electric field. A minimal mathematical model is sufficient to reproduce, and thus explain, the observed behavior. The periodic beating of the swimming hydrogel robot emerges from flutter instability thanks to the interplay between its active and passive reconfigurations in the viscous environment. Interestingly, the flutter-driven soft robot exhibits a form of electrotaxis whereby its swimming trajectory can be controlled by simply reorienting the electric field. Our findings trace the route for the embodiment of mechanical intelligence in soft robotic systems by the exploitation of flutter instability to achieve complex functional responses to simple stimuli. While the experimental study is conducted on millimeter-scale hydrogel swimmers, the design principle we introduce requires simple geometry and is hence amenable for miniaturization via micro-fabrication techniques. We believe it may also be transferred to a wider class of soft active materials.